To solve this problem, we need to identify which stage of sewage treatment is responsible for removing suspended solids. Let's analyze each option:$\newline \bullet$Secondary treatment:$\newline$ - This stage primarily involves biological processes to degrade organic matter using microorganisms.$\newline$ - It does not focus on removing suspended solids.$\newline \bullet$Primary treatment:$\newline$ - This stage involves physical processes such as sedimentation to remove large particles and suspended solids from sewage.$\newline$ - It is specifically designed to remove suspended solids.$\newline \bullet$Sludge treatment:$\newline$ - This process deals with the treatment and disposal of sludge generated during sewage treatment.$\newline$ - It is not directly involved in removing suspended solids from the sewage.$\newline \bullet$Tertiary treatment:$\newline$ - This stage involves advanced treatment processes to remove nutrients, pathogens, and other contaminants.$\newline$ - It is not primarily focused on removing suspended solids.$\newline$Based on the analysis, primary treatment is the stage that removes suspended solids.$\newline$Therefore, the correct option is Option 2: Primary treatment.